As technology evolves in the field of voice and data communications, the need for a reliable method of remotely determining with relative precision whether a fault lies on the network side or on the customer side of a communication line has become very apparent in the industry.
Currently available testing devices are unsatisfactory because they have only limited testing capabilities. As a result, certain fault conditions on the communication line cannot be detected with precision and certainty. Because of such limitations, telecommunication companies have been reluctant to make the massive investment necessary for the wide spread use of such devices on the entire network.
An important design criteria of a fault locating device is its ability to be electrically transparent on the communication line to avoid interfering with normal signal transmissions, either voice or data. This requirement has become more and more important as communication line standards are raised to allow higher data rate transmissions.
It has been clearly demonstrated that any significant parasitic effects introduced by a fault locating device in the inductive reactance, capacitive reactance, leakage resistance and series resistance of the loop can significantly degrade the quality of the signal transmission. Certain fault locating devices which have been made available during the past recent years are designed to purposely introduce termination networks on the tip and ring conductors of a telephone line to provide a characteristic impedance that is used as a unique signature. In the test mode, such devices are designed to selectively place the customer side of the telephone line in an open-circuited state. To achieve this function, four-layer diode circuits are sometimes used in serial connection with the telephone line. However, such diode circuits introduce a significant voltage drop across their terminals, in the order of 1.2 volts. If a peripheral apparatus is connected in series with the telephone line, a cumulative voltage drop of as much as 3.2 volts is produced which can adversely affect the operating limits of the telephone line.